and an osx host -- so you'll have to vary for a windows host or guest
It wasn't terribly hard last time I did this, but there was some one-time preliminary hoop-jumping. IIRC, it went somewhat as follows
First, install the Guest Additions
by clicking Devices >> Install Guest Additions
then running the appropriate program from the mounted guest additions disk
Then, add the appropriate host folder to the Shared Folders for the virtual machine:
click Settings >> Shared Folders &c&c
On the host machine, set sharing permissions for that folder, so you actually have access to it
Also add a folder to the guest machine where the shared host folder will be mounted
After going through the above one-time hoop-jumping, you still need to ensure that the host folder gets mounted when you need it. For me, something like the following worked:
http://forums.virtualbox.org/viewtopic.php?t=15868#Unfortunately, I can't double-check the above with any of my current virtual machines for various reasons (my ubuntu 11.04 beta doesn't support guest additions, my remastersys custom disk installation doesn't support guest additions, &c&c). Some details may depend on your version of virtual box
I did something like the above for a while, about a year ago, but for most purposes I find the following alternative more convenient: just use a USB stick to transport files between the guest and host. I find a mounted USB claimed by the host can be claimed by guest if dismounted from the host and conversely. You may want to check Settings >> Ports >> USB and add the stick there; you may need to click the USB icon at the bottom of the individual virtual machine screen and be sure that the particular stick is ticked. Then I just drag my host file to the USB stick, eject the stick from the host, claim the stick in the guest, and copy the file to the guest file system. Usually I have no problem mindlessly going back and forth this way, though now and then something might bork
Here's another option I haven't tried
http://www.virtualbox.org/wiki/Sharing_files_on_OSE